1. Field of the Invention
The invention relates to a method for making an aligned carbon nanotube, more particularly to a method for making an aligned carbon nanotube at a relatively low temperature.
2. Description of the Related Art
Chemical vapor deposition (referred to as CVD hereinafter) and related techniques derived therefrom, such as microwave plasma enhanced chemical vapor deposition (referred as to MPCVD hereinafter), are commonly used to make a carbon nanotube. In the CVD method or the related techniques, a porous substrate coated with a catalyst is placed in a CVD or MPCVD system. A carbon-containing reaction gas suitable for growing the carbon nanotube is introduced into the CVD or MPCVD system. The carbon-containing reaction gas is cleaved or ionized so as to react with the catalyst on the substrate and to grow the carbon nanotube.
Although the aforesaid conventional method can be used to make the carbon nanotube, the conventional method has to be conducted at a relatively high temperature and at a relatively slow reaction rate.
The reaction temperature and the reaction rate at which the carbon nanotube is made are affected primarily by the composition of the catalyst. The main function of the catalyst is to react with the carbon atoms produced by cleaving the carbon-containing reaction gas via a plasma process so as to deposit and grow the carbon nanotube. Catalysts for the growth of carbon nanotubes (CNTs) can be a single element catalyst or contain more than one element. An elemental catalyst, such as Ni or Mo, generally allows the growth of CNTs through a multi-step process to form CNTs. To improve the performance of elemental catalysts, an additive such as Mo or Y may be used. The second minor element has been found to, for example, lower the CNT growth temperature or prevent the formation of undesired graphite particles. The growth rate of the carbon nanotube is affected by the activity of the catalyst, which is restricted by the reaction temperature. The catalyst used in the conventional CVD or MPCVD process requires a relatively high catalysis temperature, which is generally higher than 550° C.
U.S. Pat. No. 6,350,488 discloses a method of synthesizing carbon nanotubes, which uses cobalt, nickel, iron or their alloys as catalysts. The CNTs growth temperature is as high as 700 to 1000° C., and the growth rate is 12 μm/min (refer to the example 1 of U.S. Pat. No. 6,350,488).
US 2002/016011 discloses a method for fabrication of carbon nanotubes film, which uses Ni/Cr as catalyst. The CNTs growth temperature is 550° C., and the growth rate is at the order of 10 nm/min (refer to the example 1 of US 2002/016011).
Therefore, it is expected to increase the growth rate of carbon nanotubes at a lower reaction temperature to reduce the complexness of fabrication procedure and hence reduce the fabrication cost.